1. Field of the Invention
This invention relates to a laser-diode-pumped solid-state laser comprising a solid-state laser rod pumped by a semiconductor laser (laser diode), and more particularly to a laser-diode-pumped solid-state laser in which the solid-state laser rod per se functions as an optical wavelength converter for converting a laser beam which is oscillated by the solid-state laser rod into a second harmonic or converting a laser beam which is oscillated by the solid-state laser rod and another laser beam into a wavelength-converted wave such as a sum frequency wave of the laser beams.
2. Description of the Prior Art
As disclosed, for instance, in SPIE, Vol. 1104, page 100, March 1989, there have been known Nd:COANP, Nd:PNP and the like as solid-state laser rods which are doped with a rare-earth element such as Nd (neodium) and have an optical wavelength-converting function. As such solid-state laser rods, Nd:LiNbO.sub.3, NYAB (Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 wherein x stands for 0.04 to 0.08) and the like are further known as disclosed in the same magazine, page 132, and these rods are referred to as "Self-Frequency-Doubling Crystal".
As the laser-diode-pumped solid-state laser in which such a solid-state laser rod is employed, there have been known those in which a NYAB crystal is employed as the solid-state laser rod and a second harmonic of the laser beam which is oscillated by the solid-state laser rod is extracted, as disclosed in SPIE, Vol. 1104, page 132, March, 1989 or LASER KENKYU, Vol. 17, No. 12, page, 48 (1989). Further, in J. Opt. Soc. Am Vol. 3, page 140 (1986), it is suggested to excite Nd:MgO:LiNbO.sub.3 by a dye laser having a wavelength of 0.60 .mu.m and to obtain a second harmonic of the laser beam oscillated by the solid-state laser rod.
Further, as disclosed, for instance, in SPIE, Vol. 1104, page 13, March, 1989, it is suggested to provide a single crystal of KTP which converts the wavelength of a laser beam oscillated by a solid-state laser in a Nd-doped YAG solid-state laser rod and a resonator, thereby obtaining a sum frequency wave of the laser beam oscillated by the solid-state laser and the pumping laser beam.
However, in conventional solid-state lasers having such a wavelength converting function, optical elements such as a nonlinear optical crystal, a solid-state laser rod, an output mirror, and an etalon or wave plate which converts the laser beam oscillated by the solid-state laser rod into a single-longitudinal-mode laser beam to stabilize the power of the wavelength-converted wave are discretely disposed and are discretely processed, lapped and coated. Accordingly, the laser beam oscillated by the solid-state laser rod is scattered at the processed surfaces and the coatings, reflected at the coatings and absorbed by the coatings, which together with the absorption in the parts results in an internal loss in the resonator as large as not less than several percents. The internal loss increases as the number of the parts increases. The internal loss reduces the power of the oscillated laser beam in the resonator and lowers wavelength conversion efficiency.
Due to the large internal loss, a high-output-power array laser is conventionally employed as the pumping light source in order to compensate for the large internal loss in the resonator and increase the power of the oscillated laser beam in the resonator, thereby improving the wavelength conversion efficiency. However, since the spectral line width of the conventional array laser is as large as several nanometers, the efficiency at which the laser beam is oscillated by the solid-state laser is low and the energy utilization efficiency is low.
Especially, since the etalon and the wave plate which are provided in the resonator in order to stabilize the power of the wavelength-converted wave much increase the internal loss, conventionally, a high-output-power array laser or broad area laser must be employed. On the other hand, when the etalon and the wave plate are eliminated, there arises a problem that the power of the wavelength-converted wave becomes unstable due to confliction between the longitudinal modes of the laser beam oscillated by the nonlinear optical element when the second harmonic of the oscillated laser beam is to be extracted.